The voltage controlled oscillator (VCO) is one of the building blocks of a frequency synthesis circuit. Discrete frequencies are accomplished with a phase-locked loop (PLL) synthesizer, which uses a VCO. The term "VCO" is used in a general sense, and includes oscillators where the frequency is under control of a current as well as those where the frequency is voltage-controlled.
Many of today's VCO's are at least partially solid state devices, having their transistor elements and some or all of their resistive, capacitive, or inductive components fabricated with semiconductor fabrication techniques. VCO circuits having all components fabricated within a single integrated circuit are referred to as "monolithic" circuits.
A problem with today's solid state VCO circuits is that they do not provide a wide frequency range with low phase noise. For applications such as cellular telephone transceivers, these characteristics are very important.
One approach to providing wide frequency range and low noise has been to use an LC oscillator circuit whose inductive or varactor elements are external to other integrated circuit components. However, this approach does not satisfy the additional need for small monolithic circuitry.